1. Field of the Invention
The present invention relates to navigation systems, and more particularly to an improved navigation and position method using Global Positioning System (GPS) receiver stations.
2. Art Background
To assist sea, air, and land navigation and other purposes, the United State Government has placed a number of satellites in orbit around the earth in such a manner that, from any point on the earth, a user operating a roving receiver from an airplane or a ship may always have a line of sight on at least four satellites. This system is referred to as the Global Positioning System (GPS). A GPS receiver receives GPS data from the satellites; from the GPS data the roving receiver can determine its position. The GPS data includes data regarding the position of the satellite. However, the GPS data is corrupted by the U.S. Government in order to degrade the accuracy of calculations performed. Such errors are easily eliminated using the proper decoding algorithms and codes; however, such information is only available to the U.S. Military. Also, atmospheric and meteorological conditions, electromagnetic interference from terrestrial sources and other satellites, kinematic motion or orientation of the plane or ship the roving receiver is located on, and other uncertainties further degrade the signals.
To ameliorate this problem, land-based reference stations at fixed, known locations have been erected to receive satellite transmissions and interpret the signals to generate measurement corrections, also referred to as DGPS (differential GPS) corrections. Using the true, known position of the receiver antenna at each reference station, these land-based reference stations derive measurement corrections that adjust the GPS data to produce more accurate results. These measurement corrections are transmitted, for example, via minimum shift keying (MSK) transmissions, to the roving receivers as deviations or offsets to be added to the measurements derived by the roving receiver from the GPS signals received directly from the satellites. An example of such a system is the Differential GPS NAVSTAR system operated by the U.S. Coast Guard to help ships navigate more accurately.
The use of reference stations has become so widespread, and the number of reference stations has grown so large, that it is quite likely that a receiver is in range of several reference stations at once. When this happens, very often the signal received from the reference station closest to the roving receiver is used to the exclusion of all others, since that signal, relative to those of the other reference stations, is usually the strongest signal and has the shortest signal path length. This method of operation reduces spatial decorrelation errors. The other measurement correction signals originating from other reference stations, although available to the roving receiver and containing measurement corrections, are generally ignored. This is justified by the fact that the likelihood of erroneous transmission increases with distance, and that the spatial decorrelation errors in the correction increase with distance.